The following description of the background of the invention is provided solely to aid the understanding of the reader. None of the information provided or references cited is admitted to be prior art to the invention.
Cellular signal transduction is a fundamental mechanism whereby external stimuli that regulate diverse cellular processes are relayed to the interior of cells. One of the key biochemical mechanisms of signal transduction involves the reversible phosphorylation of proteins, which enables regulation of the activity of mature proteins by altering their structure and function. For reviews, see Posada and Cooper, Mol. Biol. Cell, 3:583-392 (1992) and Hardie, Symp. Soc. Exp. Biol. 44:241-255 (1990)). The best characterized protein kinases in eukaryotes phosphorylate proteins on the alcohol moiety of serine, threonine and tyrosine residues. These kinases largely fall into two groups, those specific for phosphorylating serines and threonines, and those specific for phosphorylating tyrosines. The tyrosine kinases can be further divided into receptor and non-receptor proteins.
Protein kinases are one of the largest families of eukaryotic proteins with several hundred known members. Alignment of primary peptide sequences of these proteins shows that they share a 250-300 amino acid domain that can be subdivided into 12 distinct subdomains (I-XII) that comprise the common catalytic core structure. These conserved protein motifs have recently been exploited using PCR-based cloning strategies leading to a significant expansion of the known kinases. Multiple alignment of the sequences in the catalytic domain of protein kinases and subsequent phylogenetic analysis permits their segregation into a phylogenetic tree. In this manner, related kinases are clustered into distinct branches or subfamilies including: tyrosine kinases, cyclic-nucleotide-dependent kinases, calcium/calmodulin kinases, cyclin-dependent kinases and MAP-kinases, as well as several other less defined subfamilies. (See Hanks and Hunter, FASEB J. 9:576-595 (1995).
Receptor tyrosine kinases (RTKs) belong to a family of transmembrane proteins and have been implicated in numerous cellular signaling pathways. The predominant biological activity of some RTKs is the stimulation of cell growth and proliferation, while other RTKs are involved in promoting differentiation. In some instances, a single tyrosine kinase can inhibit or stimulate cell proliferation depending on the cellular environment in which it is expressed. RTKs are composed of at least three domains: an extracellular ligand binding domain, a transmembrane domain and a cytoplasmic domain containing at least one enzymatic domain capable of phosphorylating tyrosine residues. Ligand binding to membrane bound receptors induces the formation of receptor dimers and allosteric changes that activate the intracellular kinase domains and result in the self-phosphorylation (autophosphorylation and/or transphosphorylation) of the receptor on tyrosine residues. RTKs are also known to form heterodimers. A possible role for receptor heterodimerizaion is described in Carraway and Cantley, Cell 78:5-8 9 (1994).
The non-receptor tyrosine kinases do not contain a transmembrane domain or an extracellular domain and share non-catalytic domains in addition to sharing their catalytic kinase domains. Such non-catalytic domains include the SH2 domain (Src homology domain 2) and SH3 domains (Src homology domain 3). The non-catalytic domains are thought to be important in the regulation of protein-protein interactions during signal transduction.
Receptor tyrosine kinases are known to play a role in the proliferation, differentiation and/or survival of many cell types. One example is the Trk family of receptors. The Trks are receptors for several known neurotrophic factors including nerve growth factor (NGF). Binding of NGF to TrkA induces phosphorylation of the receptor and subsequent differentiation of the PC12 pheochromocytoma cell line, a model for neuronal development. (Kaplan, et al, Science 252:554-558 (1991); Yan, et al, Science 252:561-563 (1991)). Other members of the Trks family include TrkB and TrkC, which are expressed in a variety of structures in nervous system and respond to binding of other neurotrophic factors such as brain-derived neurotrophic factor and neurotrophin-3 (Klein, et al Development 109:845-850 (1990); Glass, et al Cell 66:405-413 (1991); Klein, et al Cell 66:395-403 (1991)).
Several RTKs and growth factors were originally identified as activated oncogenes (Aaronson, Science 254:1146-1153 (1991); Bishop, Cell 64:235-248 (1991)) and there has long been a belief that some RTKs may be involved in the development of cancers. Several studies appear to support this notion. These include the high correlation of RTK overexpression with certain human cancers including HER2 with breast and ovarian cancers (Slamon, et al., Science 235:177-182 (1987)), PDGF and its receptors with a high fraction of sarcomas and glially derived neoplasms, and EGF-R with squamous cell carcinomas and glioblastomas (reviewed in Aaronson, (1991)).
Several RTKs have been associated with the growth and development of lung cancer cells including c-kit (Hida, et al Int. J. Can. 0 (supp 8):108-109 (1994); Krystal, et al, Can. Res. 56(2);370-376 (1996)), trk (Oelmann, et al Can. Res. 55(10):2212-2219 (1995)), Her2/neu (Tsai, et al Can. Ras. 56 (5):1068-1074 (1996)) and EGF-R (Moody, Peptides 17(3):545-555 (1996)). The identification of a lung cancer specific RTK would be advantageous for the development of specific drugs that could inhibit the signal transduction activity of the RTK thereby suppressing the RTK driven growth of the cancer.